Apparatus for improving rotary drilling operations

ABSTRACT

Apparatus for drilling a well bore using a rotary drill string provided with a drill collar and a drill bit on the lower end of the drill string. The drill collar has a reduced diameter annular groove formed on its exterior surface adjacent the threads on either or both the pin end and box end thereof. The amount of the reduction in diameter of each groove is such as to reduce the Moment of Inertia from about 50 to 80 percent of the original cross-section of the drill collar. The length of each groove is selected so as to minimize the stress concentration due to discontinuity produced by the change in cross-section of the drill collar while maintaining the effectiveness of the weight of the drill collar.

United States Patent 1 [11] 3,730,286

' [21] Appl. No.: 267,713

Weiner May 1, 1973 APPARATUS F OR IMPROVING FOREIGN PATENTS OR APPLICATIONS ROTARY DRILLING OPERATIONS 269,777 4/ 1969 Austria ..175/320 [75] Inventor: Peter D. Weiner, Bryan, Tex, 1,317,815 1/1963 France ..175/320 0 1,026,651 4/1966 Great Britain ..175/320 [73] Assignee: Esso Production Research Company, 159,777 1 H1962 U.S.S.R. ..l75/320 l Houston, Tex.

[22] Filed: June 1972 I Primary Examiner-David H. Brown Attorney-Melvin F. Fincke et a1.

[57] ABSTRACT Related US. Application Data [63] Continuation of Ser. No. 52,404, July 6, 1970, abanstring provided with a drill collar and a drill bit on the dofledlower end of the drill string. The drill collar has a reduced diameter annular groove formed on its exteri- [52] US. Cl. ..175/320, 154/! R r rfa adj cent the threads on eith r or o h the 51 Int. Cl. ..E21b 17/00, E216 15/00 pin end and box end thereof- The amount of h [58] Field of Search ..175/320; 64/1 R, reduction in diameter of each groove is Such as to 64 S; 285/] 14 reduce the Moment of Inertia from about 50 to 80 Apparatus for drilling a well bore using a rotary drill percent of the original cross-section of the drill collar.'

[56] References Cited The length of each groove is selected so as to minimize the stress concentration due to discontinuity 'UNTED STATES PATENTS produced by the change in cross-section of the drill collar while maintaining the effectiveness of the 2,676,820 954 BOICB ..285/1 l4 weight f the collar. 3,061,024 10/1962 Thompson.. ....l75/32O X 3,152,458 10/1964 Simonin 1 75/320 X 18 Claims, 6 Drawing Figures 3,637,033 l/197 2 Mayall ..175/320 l 10 DRILL BIT 3 bi e l l3 35 AL I6 18 Patented May 1, 1973 3,730,286

2 Sheets-Sheet 1 v INVENT PETER D. WEIN ATTORNEY.

S ksi Patented May 1, 1973 2 Sheets-Sheet 2 I I I I I I I PIN STRESS 22- P |4 I I L I2 I I I I I I I l 5 0 2o so so oo '10 PERCENT I-(%) I Z ALTERNATING sTREss vs PERCENT MOMENT OF INERTIA FIG. 5. 5

I05 I0 1 L 25 I I I .5- FIG. 6.

o I I I I I II I I I I v I I05 I06 I07 INVENTOR.

I 'F PETER 0. WEINER, ALTERNATING STRESS VS I IFE B A T TORNEY BACKGROUND OF THE INVENTION Increasing drilling depths in rotary drilling operations and the associated increases in the magnitude and failures. The rigidity of the drill collar connection is one cause of such failures. i

Boreholes often deviate in excess of per 100 feet from the vertical. As the drill string follows these deviations the drill collars undergo cyclic loading which varies from maximum tension to compression and back again to tension. There are normally three or more drill collars on a drill string, each being connected to the other by a threaded connection and since the connections are weak compared to the rest of the collar, most of the bending occurs in the joints. When fatigue failures of drill collars occur, it is necessary to fish any separated pieces from the hole when possible and when that is not possible it'may be necessary to abandon the hole altogether.

The present invention concerns eliminating or at least decreasing the rate of occurrence of drill collar connection failures. The drill collar connection is made more flexible by modifying the exterior surface of the drill collar section in the vicinity of the connection. A highly flexible surface is formed on the drill collar by turning a groove on the drill collar section adjacent the threads on the pin or box end thereof or both. The depth and length of the annular groove are predetermined in order to achieve a desired flexibility and strength while maintaining the weight function of the drill collar. r

SUMMARY OF THE INVENTION Briefly, the present invention may be described as apparatus for improving rotary drilling operations comprising a drill collar sectionhaving a threaded pin end and a threaded box end and a reduced diameter annular groove formed on said drill collar section adjacent the threads of atleast one of said ends, the amount of said reduction in diameter being such that the Moment of Inertia is reduced in the range from 50 to 80 percent, and preferably 65 to 75 percent of the original cross section of the drill collar.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view illustrating two drill collars modified in accordance with the teachings of the invention con- I nected together;

FIG. 2 is a view taken on lines 2-2 of FIG. 1;

FIG. 3 is a schematic illustration of fatigue test apparatus showing the drill collars mounted in the test apparatus and under test;

FIG. 4 is a longitudinal view of a curved portion [of a well bore with the modified drill collars connected to the lower end of a drill string positioned in the well bore; i

FIG. 5 is a graph which shows the alternating stress in the pin 8, and in the box 8,, as a function of the percent Moment of Inertia, %I; and

FIG. 6 is a graph which indicates the increased life of the modified drill collar obtained by reducing the alternating stress in the pin and in the box.

DESCRIPTION THE PREFERRED EMBODIM ENT S In FIGS. 1 and 2 a drill collar 10 having a pin end 11 is threaded to the box end 12 of another drill collar 13. Drill collar 10 has been modified by an annular groove 15 turned on the drill collar adjacent the end 16 of the threads on pin 11 and drill collar 13 has been modified by an annular groove 17 turned on the drill collar adjacent the end 18 of the threads on box 12.

Drill collar fatigue tests were conducted to verify means for increasing the life of drillcollars subjected to cyclic bending loads. Such tests determined the reduction in cyclic stresses produced in a drill collar joint in which an external relief groove was formed adjacent the threads of such joint. This groove on one or both sides of the connection was introduced to enable the collar to bend in the grooves, point or section of low mean stress, rather than in the threads of the joint which contain high mean stresses produced by joint make-up. The drill collar tests were conducted on the test machine illustrated in FIG. 3 which imposed a constant bending moment across the joint simulating a dog leg. There is shown in F IG. 3 spaced-apart supports 20 and 21 on each of which is mounted a bearing denoted 22 and 23, respectively, in which the ends of drill collars 10 and 13 are arranged. Between supports 20 and 21 is positioned a loading frame 25 on which are mounted additional bearings 26' and 27 which support pipe collars 10 and 13, respectively, adjacent the threaded connection thereof. A hydraulic jack or hydraulic cylinder 28 moves loading frame 25 upwardly which moves bearings 26 and 27 upwardly to place a bending moment on the pipe collars 10 and 13. A motor 29 drives a shaft 30 by belt drive means 31 to rotate pipe collars 10 and 13 in bearings 22, 26, 27 and 23. The dotted lines show the position of the pipe collars and bearings 26 and 27 as the pipe collars are rotated. r

Strain gages were affixed to critical areas indicated by theletter S in FIG. 1 to determine the stresses induced in the drill collars when negotiating the dog leg simulated'by the test machine. The gages used were standard 120 ohm resistance precision strain gages sold by Micro Measurements, Inc. bonded to the collars with Eastman 910 adhesive using; normally acceptable strain gage techniques. The gages were monitored during make-up and at various times during the cycling of the drill collars on the test machine. Cycle counting was done with a magnetic pick-up connected to an electronic counter. Periodically the machine was stopped and the strain gages were read with a Bean- Digital Strain Indicator and switch and balance unit. No dynamic readings were taken. In order to correlate data from the'various joints tested: the drill collars were run at a specified deflection 0.177 inches corresponding to an angle of deviation of 10 per feet which is more than a drill collar would normally experience in practiceuThe specified deflection was later increased to 0.231 inches corresponding to a 13 per 100 feet The critical areas at which the strain gages were located were at the base of the pin and over the last engaged thread on the box. The stress concentrations caused by unengaged threads and external grooves made these areas critical. The gages were laid 180 apart at each location for reliability of readings and protection against failure of a gage. To determine the stress at the given location an arithmetic mean of the two gages was found. These were averaged and the stresses calculated by S e E, where S stress in psi e strain in micro inches per inch E modulus of elasticity in psi.

This equation is valid since only a bending load was applied to the test collar.

In FIG; 5, there is plotted the alternating stress in the pin 5,, and in the box 5,, as a function of the percent Moment of Inertia, which is defined as I/I X 100 where: 1 is the Moment of Inertia of a standard collar with no external relief and I is the Moment of Inertia of a modified collar with external relief.

The Moment of Inertia, l=(1r/64) [D,,- D,] where:

D, Outside diameter of drill collar D, Inside diameter of drill collar.

As seen in FIG. 5 the curves show that the Moment of of Inertia should be reduced in the range of from about 50 to 80 percent in order to achieve significant reduction of alternating stresses in the pin and box and preferably in the range of from about 65 to 75 percent to achieve optimum reduction.

The graph shown in FIG. 6 plots alternating stress versus life cycles which'emphasizes the practical importance of reduction of the pin and box stresses.

The test data from which the curves of FIGS. 5 and 6 were plotted show, for example, that a 4 A; inch O.D. A.P.l. drill collar connection with no relief collar failed when subjected to 393,000 life cycles on the test machine under a deflection of 0.177 inches whereas a similar collar connection under similar test conditions achieved more than 5,500,000 life cycles when relief grooves, each 3 Vs inch Drg, (1 inch reduction in diameter) were formed on the pin section and box section of the drill collar connection. In these tests, referring to FIG. 1, L, the distance from the shoulder on the pin end to the edge of the groove was 1 inch and the distance L,,, from the end of the internal taper to the edge of the groove on on the box section was 1 inch and the length of each (relief) groove, Lrg, was 3 inches.

The cross-sectional area at the relief groove should not be less than the cross-sectional area at the last engaged pin thread. Any further reduction risks a tensile or torsional failure in the relief groove. However, the

life of the connection in bending would be increased even ifthe cross-section is less than the diameter of the last engaged threads.

The external groove should be formed as close to the threads of the pin or box as possible. As shown in FIG. I groove commences 1 inch from the shoulder at the end 16 of the pin threads and groove I7 commences 1 inch from the point where the box assumes the internal diameter of the drill collar. The groove may be commenced from about I to about 24 inches from the end of the threads of the pin or box sections. The length of a the groove is selected so as to minimize the stress con centration due to discontinuity produced by the change in the cross-section of the drill collar while at the same time ensuring that the effectiveness of the drill collar (weight on drill string) is notsignificantly reduced. The length of the external groove should preferably be at least equal to the outside diameter of the drill collar in order to reduce the stress concentrations produced by the change in cross-section if the diameter'of the drill collar is 6 or more inches. Although likelihood of failure at the change in cross-section would increase, the length of the groove may be made less than the outside diameter of the drill collar or 6 inches minimum. The external groove should be provided with a generous fillet as indicated at to permit but very small stress concentration at the change in diameters.

In FIG. 4 there is shown a curved portion of a borehole in which is positioned a drill string 41, the

. lower end of which is made-up of attached drill collars 10 and 13. This figure illustrates the importance of providing flexibility in the drill collars at the joint connection between the drillv collars indicated at 42 to decrease or eliminate stresses in the joint resulting from rotation of the drill collars and bending caused by the curvature of the borehole.

More than two annular grooves may be formed in each drill collar. The principle of this invention may be applied to other components similar to pipe collars such as drill string stabilizers. Other modifications may be made to the embodiments of the invention described herein within the scope of the appended claims.

Having fully described the apparatus, operation, objects, and advantages of my invention, I claim:

1. Apparatus for improving rotary drilling operations comprising:

a drill collar section for connection .into a drill string having a threaded pin end and a threaded box end and a reduced diameter annular groove formed on the outer surface of said drill collar adjacent the threads of at least one of said ends, the amount of said reduction in said diameter being such that the Moment oflnertia is reduced in the range from about 50 to 80 percent of the original cross-section of the drill collar, said groove being provided with a fillet at each end thereof to reduce stress concentration at the change in diameters.

2. Apparatus as recited in claim 1 in which the amount of reduction 'in said diameter is such that said Moment of Inertia is reduced in the range from about 65 to percent of the original cross-section of the drill collar.

3. Apparatus as recited in claim 1 in which the length of each groove is selected so as to limit the stress concentration due to discontinuity produced by the change in cross-section of the drill collar while maintaining the effectiveness of the weight of the drill collar.

4. Apparatus as recited in claim 3 in which the crosssectional area of said groove is not less than the crosssectional area at the last engaged pin thread.

5. Apparatus as recited in claim 4 in which the length of said groove is at least 6 inches.

6. Apparatus as recited in claim 4 in which the length of said groove is at least equal to the outside diameter of said drill collar.

7. Apparatus as recited in claim 4 in which said groove is formed adjacent the threads'at each end of said drill collar.

8. Apparatus as recited in claim 7 in which each groove is commenced about 1 inch from the threads on said pin end or box end of said drill collar.

9. Apparatus as recited in claim 7 in which each groove is commenced about 1 to 24 inches from the threads on pin end or box end of said drill collar.

10. Apparatus for drilling a well bore comprising:

a drill string;

a plurality of drill collars arranged in said drill string at the lower end thereof;

a rotary drill bit attached to the lower end of said drill collars;

at least one drill collar having a threaded pin end and a threaded box end and a reduced diameter annular groove formed on said drill collar adjacent the threads of at least one of said ends, the amount of said reduction in diameter being such that the Moment of Inertia is reduced in the range from about 50 to 80 percent of the original cross-section of the drill collar, said groove being provided with a fillet at each end thereof to reduce stress concentration at the change in diameters.

11. Apparatus as recited in claim 10 in which the amount of reduction in said diameter is such that said Moment of Inertia is reduced in the range from about 65 to percent of the original cross-section of the drill collar.

12. Apparatus as recited in claim 10 in which the length of each groove is selected so as to limit the stress concentration due to discontinuity produced by the change in cross-section of the drill collar while maintaining the effectiveness of the weight of the drill collar.

13. Apparatus as recited in claim 11 in which the cross-sectional area of said groove is not less than the crosssectional area at the last engaged pin thread. r

14. Apparatus as recited in claim 12 in which the length of said groove is at least 6 inches.

15. Apparatus as recited in claim 13 in which the length of said groove is at least equal to the outside diameter of said drill collar.

16. Apparatus as recited in claim 13 in which said groove is formed adjacent the threads at each end of said drill collar.

17. Apparatus as recited in claim 16 in which each groove is commenced about 1 inch from the threads on said pin end or box end of said drill collar.

18. Apparatus as recited in claim 16 in which each groove is commenced about 1 to 24 inches from the threads on pin end or box end of said drill collar.

Disclaimer and Dedication 3,730,286.Petc9" D. Weinea", Bryan, Tex. APPARATUS FOR IMPROVING ROTARY DRILLING OPERATIONS. Patent dated May 1, 1973. Disclaimer and dedication filed June 27, 1974:, by the assignee, Esso Pwoductzbn Reseamh Company. Hereby disclaims and dedicates t0 the Public the remaining term of said patent.

[Ofiicz'al Gazette May 20, 1975.] 

1. Apparatus for improving rotary drilling operations comprising: a drill collar section for connection into a drill string having a threaded pin end and a threaded box end and a reduced diameter annular groove formed on the outer surface of said drill collar adjacent the threads of at least one of said ends, the amount of said reduction in said diameter being such that the Moment of Inertia is reduced in the range from about 50 to 80 percent of the original cross-section of the drill collar, said groove being provided with a fillet at each end thereof to reduce stress concentration at the change in diameters.
 2. Apparatus as recited in claim 1 in which the amount of reduction in said diameter is such that said Moment of Inertia is reduced in the range from about 65 to 75 percent of the original cross-section of the drill collar.
 3. Apparatus as recited in claim 1 in which the length of each groove is selected so as to limit the stress concentration due to discontinuity produced by the change in cross-section of the drill collar while maintaining the effectiveness of the weight of the drill collar.
 4. Apparatus as recited in claim 3 in which the cross-sectional area of said groove is not less than the cross-sectional area at the last engaged pin thread.
 5. Apparatus as recited in claim 4 in which the length of said groove is at least 6 inches.
 6. Apparatus as recited in claim 4 in which the length of said groove is at least equal to the outside diameter of said drill collar.
 7. Apparatus as recited in claim 4 in which said groove is formed adjacent the threads at each end of said drill collar.
 8. Apparatus as recited in claim 7 in which each groove is commenced about 1 inch from the threads on said pin end or box end of said drill collar.
 9. Apparatus as recited in claim 7 in which each groove is commenced about 1 to 24 inches from the threads on pin end or box end of said drill collar.
 10. Apparatus for drilling a well bore comprising: a drill string; a plurality of drill collars arranged in said drill string at the lower end thereof; a rotary drill bit attached to the lower end of said drill collars; at least one drill collar having a threaded pin end and a threaded box end and a reduced diameter annular groove formed on said drill collar adjacent the threads of at least one of said ends, the amount of said reduction in diameter being such that the Moment of Inertia is reduced in the range from about 50 to 80 percent of the original cross-section of the drill collar, said groove being provided with a fillet at each end thereof to reduce stress concentration at the change in diameters.
 11. Apparatus as recited in claim 10 in which the amount of reduction in said diameter is such that said Moment of Inertia is reduced in the range from about 65 to 75 percent of the original cross-section of the drill collar.
 12. Apparatus as recited in claim 10 in which the length of each groove is selected so as to limit the stress concentration due to discontinuity produced by the change in cross-section of the drill collar while maintaining the effectiveness of the weight of the drill collar.
 13. Apparatus as recited in claim 11 in which the cross-sectional area of said groove is not less than the cross-sectional area at the last engaged pin thread.
 14. Apparatus as recited in claim 12 in which the length of said groove is at least 6 inches.
 15. Apparatus as recited in claim 13 in which the length of said groove is at least equal to the outside diameter of said drill collar.
 16. Apparatus as recited in claim 13 in which said groove is formed adjacent the threads at each end of said drill collar.
 17. Apparatus as recited in claim 16 in which each groove is commenced about 1 inch from the threads on said pin end or box end of said drill collar.
 18. Apparatus as recited in claim 16 in which each groove is commenced about 1 to 24 inches from the threads on pin end or box end of said drill collar. 